Dimensional tolerance and surface finish in machining are widely known to be related directly to the deflection of the cutting tool or the workpiece, or both, steady or vibratory, which in turn is a matter of the rigidity of the system in relation to the magnitude of the loads imposed thereon by the cutting action, whether the operation be drilling, milling, boring, turning, grinding, broaching, or combinations thereof, e.g., so-called "turn broaching". The time-honored shop practice of taking a rough cut to remove material and a finish cut, one or more, for size and surface finish is in part a reflection of the lack of rigidity of the machining system overall, "system" in this sense meaning not merely the cutting tool and the frame of the machine that wields the tool and supports the work, but also the workpiece itself.
The workpiece is necessarily a variable in a general machining situation, although less so in repetitive production machining operations, such, for example, as transfer lines and similarly automated production machining operations.
In either case, the approach heretofore generally taken to the realization of greater rigidity in the machine tool per se, whether for the sake of applying greater cutting forces, or achieving tighter machining tolerances or better finishes, has been toward more massive framing in the traditional format, whether of the freestanding column sort identified with so-called "machining centers" having horizontal spindles, the C-frames of smaller vertical spindle machines, or the column-spanning beams of so-called portal and gantry machines.
A noteworthy departure appeared in 1987 with the public announcement in Great Britain of a machine having a tetrahedral frame fitted out as a grinder said to be able to maintain nanometric tolerances. Photographs published in the London Times of 3 January 1988, as well as in 1987 literature of the National Physical Laboratory, show a tetrahedral frame of six stout struts each connected at its ends to a large ball at each of the four nodes of the frame, and each having a mid-length load-receiving connection to the work support or to the tool-spindle support. As the published prototype was constructed on a relatively small dimensional scale, the tubular struts incorporated vibration-damping and were tuned to a natural frequency well above the level of the forcing frequencies to be anticipated.
The tetrahedral frame is the subject of U.S. Pat. No. 4,872,291 of Oct. 10, 1989.
The practical limitations of the tetrahedral frame, due in large part to its lack of orthogonal symmetry about the tool-workpiece interface, to its poor efficiency of plant space utilization for the limited usable space within its confines, and to its lack of adequate facility for transferring forces from tool and work supports to the frame without flexural stress in the frame, have led to the present invention.